1. Technical Field
The present invention relates to a motor drive device including an inverter circuit driven by a PWM (Pulse Width Modulation) signal, and in particular, to a technique for protecting a fail safe semiconductor switching element arranged between an inverter circuit and a motor from a back electromotive force generated by the motor.
2. Related Art
In an electrical power steering device of a vehicle, an electrical motor such as a three-phase brushless motor is arranged to apply a steering assisting force corresponding to the steering torque of a handle to a steering mechanism. A motor drive device according to the PWM control method is known for the device for driving the motor.
The motor drive device of the PWM control method generally includes an inverter circuit driven by the PWM signal having a predetermined duty. The inverter circuit is configured by a so-called three-phase bridge in which three sets of a pair of upper and lower arms each having a semiconductor switching element on the upper arm and the lower arm are arranged. The voltage of each phase is supplied from the inverter circuit to the motor when each switching element is ON/OFF operated based on the PWM signal thereby driving the motor.
A motor drive device in which a fail safe mechanical relay is arranged between the inverter circuit and the motor to prevent a current from flowing from the inverter circuit to the motor (or from the motor to the inverter circuit) when failure of the circuit is detected is also known (e.g., Japanese Patent Publication No. 3686471 and Japanese Unexamined Patent Publication No. 2005-199746).
Such failure of the circuit includes various failures that occur in the motor drive device. For instance, the failure may be an ON failure in which each switching element of the inverter circuit does not change from the ON state to the OFF state and remains in the ON state, or a short circuit failure that occurs at the wiring portion other than in the switching element. In addition, there may be a failure in which an abnormality occurs inside the CPU that controls the inverter circuit and the CPU carries out a control different from the original control.
When the above-mentioned failure of the circuit is detected, a control in which the switching elements of the inverter circuit are all set to the OFF state or a control in which a power supply relay arranged between the inverter circuit and the vehicle battery is set to the OFF state is carried out. The power supply to the inverter circuit and the motor is thereby stopped, so that breakage of the inverter circuit, false operation such as assistance being carried out in the direction not predicable by the driver, or the like can be prevented.
When the failure of the circuit is detected, the driver steers the handle by human power since the steering assisting force by the motor is not applied. In this case, the motor also turns in accordance with the steering of the handle, and the motor functions as a power generator. Therefore, a large resistance is applied on the handle operation by the power generating operation of the motor when the motor and the inverter circuit remain electrically connected. That is, a great amount of force will be required to turn the handle. The fail safe mechanical relay for electrically disconnecting the inverter circuit and the motor is thus installed as in Japanese Patent Publication No. 3686471 and Japanese Unexamined Patent Publication No. 2005-199746 to prevent such drawback.
However, in the electrical power steering device, there is a demand to further miniaturize the control circuit although the supply power to the motor is large since the control circuit is to be mounted in the vehicle. Therefore, if the mechanical relay is used as in Japanese Patent Publication No. 3686471 and Japanese Unexamined Patent Publication No. 2005-199746, the relay itself becomes large and the above-mentioned demand cannot be met. A motor drive device in which a fail safe semiconductor switching element is arranged between the inverter circuit and the motor in place of the mechanical relay as described in Japanese Unexamined Patent Publication No. 2009-274686 is thus known (e.g., Japanese Unexamined Patent Publication No. 2009-274686).
In the motor drive device of Japanese Unexamined Patent Publication No. 2009-274686, an FET (Field Effect Transistor) is arranged on a power supply line between the motor and the inverter circuit, where an FET group of each power supply line and an FET group configuring the inverter circuit are both turned OFF when an abnormality such as a short-circuit occurs in the inverter circuit.
With this configuration, however, if the FET of the power supply line is turned OFF while current is flowing to the motor when the abnormality occurs, back electromotive force originating from the inductance of the motor is generated and applied on the FET as a spike voltage (instantaneous large voltage). The FET thus may break.
In Japanese Unexamined Patent Publication No. 2009-220705, a technique of protecting the semiconductor switching element from the spike voltage in the motor drive device in which a fail safe FET is arranged on the power supply line between the motor and the inverter circuit is proposed. In Japanese Unexamined Patent Publication No. 2009-220705, when the abnormality occurs, all the FETs of the inverter circuit are turned OFF, and then the current value of each phase is detected and the FET is sequentially turned OFF from the phase in which the current value becomes smaller than or equal to a predetermined reference value rather than turning OFF the fail safe FETs on the power supply line all at the same time. The spike voltage thus can be suppressed and the breakage of the FET can be prevented.
In the motor drive device of Japanese Unexamined Patent Publication No. 2009-220705, the fail safe FET of the phase in which the current value is smaller than or equal to a predetermined value is sequentially turned OFF to disconnect the inverter circuit and the motor when an abnormality occurs, and hence the current value flowing to each phase needs to be detected on a constant basis. Thus, a current detection element such as a shunt resistor is to be arranged for each phase, which enlarges the circuit. The control content is also complex as the current value of each phase is to be compared with the reference value and the OFF control is to be carried out for every fail safe FET, which leads to false operation of the circuit and makes the circuit more complex.